... is the chain rule. Straight continuous lines differentiate downwards (integrate up) with respect to the main variable (in this case ), and the straight dashed line similarly but with respect to the dashed balloon expression (the inner function of the composite which is subject to the chain rule).

Then, working backwards through the chain rule to integrate the whole of the original fraction...

... where (key in spoiler) ...

Spoiler:

The general drift is...

Spoiler:

Use the same diagram to check the integration by differentiating your result (i.e. by working down/clockwise instead of up/anti-clockwise through the same picture).
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